Lactic acid production, separation and/or recovery process

ABSTRACT

A process for the production and isolation of lactic acid is provided. A lactate feed solution, preferably obtained from a fermentation broth is combined with and extracted by a water-immiscible base in the presence of an acidifying agent. Lactic acid is recovered from the resulting organic phase. Recovered carbonate or bicarbonate from the aqueous phase can be recycled to the fermentor and regenerated extractant can be recycled for use in the extraction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/556,384,filed Nov. 13, 1995, now U.S. Pat. No. 6,187,951 which is acontinuation-in-part of application Ser. No. 08/207,773, filed Mar. 8,1994, now U.S. Pat. No. 5,510,526, which is a continuation-in-part ofapplication Ser. No. 08/084,810, filed Jun. 29, 1993, which isabandoned, which application(s) are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the production, separationand/or recovery of lactic acid and more particularly to the production,separation and recovery of lactic acid via a fermentation process andthe separation and/or recovery of lactic acid from a lactate feedsolution such as is obtained from a fermentation broth or other sources.

2. Description of the Prior Art

Lactic acid has long been used as a food additive and in variouschemical and pharmaceutical applications. More recently, lactic acid hasbeen used in the making of biodegradable polymers both as a replacementfor present plastic materials as well as various new uses wherebiodegradability is needed or desired. Accordingly, there is an everincreasing demand for lactic acid. The present invention aims at meetingthis demand by providing an efficient and environmentally friendlyprocess for producing lactic acid, that can, if desired, be employed toavoid the consumption of bases and acids; and, that can be used, ifdesired, to substantially reduce, if not eliminate, the formation ofwaste or byproduct salts.

Production of lactic acid is commonly carried out by fermentation of astrain of the bacterial genus Lactobacillus and more particularly by thespecies Lactobacillus delbrueckii or Lactobacillus acidophilus asexamples. In general, the production of lactic acid by fermentation in afermentation broth is well known. The fermentation substrate consists ofcarbohydrates together with suitable mineral and proteinaceousnutrients. Because the lactic acid producing microorganisms areinhibited in a strongly acidic environment, the pH of the fermentationbroth must be kept above 4.5, and preferably within the range of about5.0 to 7.0, more preferably within the range of about 5.5 to 6.5, andmost preferably within the range of about 6.0 to 6.5. To maintain thispH level, suitable water-soluble basic substances or agents that arenon-toxic to the acid producing microorganism, such as alkali metalhydroxides, carbonates or bicarbonates or alkaline earth metalhydroxides or carbonates, are commonly added to the fermentation brothto neutralize the acid being produced. This results in the formation ofa lactate solution rather than the desired lactic acid product. Suchlactate solution contains the lactate anion and the corresponding cationof the substance used to neutralize the fermentation broth.

Various methods have been proposed for the recovery of lactic acid froma fermentation broth. Where the fermentation is carried out in thepresence of calcium carbonate, it is possible to recover the lactic acidby acidification with sulfuric acid. This results in the precipitationof calcium sulfate, while free lactic acid remains in the mother liquor.If desired, the mother liquor may be concentrated to up to about 90% byweight lactic acid. Subsequently, lactic acid may be extracted from themother liquor with a suitable organic extractant, to yield an extractwhich is later back-extracted with water, or the acid may be adsorbed ona suitable adsorbent and later desorbed. The resulting aqueous lacticacid solution may then be concentrated. This method has thedisadvantage, however, that it irreversibly consumes calcium carbonateand sulfuric acid and leaves, as waste, large quantities of calciumsulfate, which can give rise to disposal problems.

U.S. Pat. No. 5,132,456 (King et al.) describes a process for recoveringcarboxylic acid from a carboxylic acid-containing aqueous feed streamhaving a pH close to or above the pK_(a) level of the acid. Inaccordance with that process the recovery involves what may be describedas a cascade type acid withdrawal operation in which the basicity of theextractant is increased stepwise. In a first stage of the process, thefeed stream is contacted with an adsorbent such as a strongly basicextractant or a solid anion exchanger. In a second stage the acid-loadedadsorbent is contacted with an aqueous solution of ammonia or a lowmolecular weight trialkyl amine having a stronger affinity to thecarboxylic acid that is being recovered than the adsorber used in thefirst stage. In this way the aqueous solution of a water-solublecarboxylic acid ammonium salt is formed. This is then subjected to heattreatment, whereby the salt is decomposed to yield back the trialkylamine or ammonia and free carboxylic acid. Applying this process tolactic acid involves the formation of salts of lactic acid with strongbases having a pK_(a) value of about 9-11; i.e. the conjugate acid ofthe base has a pK_(a) of 9-11. Thus, the decomposition of these saltsinto free lactic acid is energy intensive. Examples 12-14 of the '456patent mention the use of Alamine 336 (tricaprylylamine) for theextraction of, among others, lactic acid from an aqueous solution, butno yields are mentioned. Upon the extraction of even small quantities oflactic acid from a fermentation broth the pH of the broth rises rapidlyto above 7. As shown in FIGS. 3 and 4 of the '456 patent, the uptake ofcarboxylic acids from aqueous solutions drops rapidly with an increaseof the pH. It is, therefore, inherent in these examples that the lacticacid uptake, if any, is negligible. It is further noted that upon heattreatment and concentration of an ammonium lactate, crystalline lacticacid does not precipitate and instead the viscosity of the solutionsincreases steadily as a result of self-association of the acid. It isthus evident that the process of U.S. Pat. No. 5,132,456 is unsuitablefor the recovery of lactic acid from a fermentation broth.

U.S. Pat. Nos. 4,444,881 and 4,405,717 (Urbas) describe a process forthe recovery of an organic acid from a diluted aqueous solution of itscalcium salt by adding a water-soluble trialkyl amine carbonate to thesolution to form on the one hand a water soluble trialkyl ammonium saltof the acid, which salt remains in solution, and on the other handcalcium carbonate which precipitates. After removal of the calciumcarbonate the remaining mother liquor is heated for the separaterecovery of the amine and the product acid. The decomposition of thetrialkylammonium salts of this reference into free acids is energyintensive.

U.S. Pat. No. 4,282,323 (Yates) describes a process for obtaining lowercarboxylic acids from a salt solution of such carboxylic acid asobtained from fermentation. The process appears to be applicable to arestricted number of lower aliphatic and aromatic monocarboxylic acidsand is specifically described only in relation to acetic acid. Inaccordance with that process, the aqueous solution of a carboxylic acidsalt is concentrated in the presence of a liquid polar organic solventserving as extractant, with pressurized carbon dioxide, to convert atleast part of the salt to the corresponding free acid which is taken upby the organic phase, from which it is subsequently recovered. It isinherent in the use of a polar organic extractant that the bulk of thecarboxylic acid remains in the neutral to basic aqueous phase, andindeed the recovery rates reported in U.S. Pat. No. 4,282,323 are low,ranging between 4.8% and 18% of the acid initially present.

U.S. Pat. No. 4,275,234 (Baniel et al) is directed to a method ofrecovering various acids in their free form from aqueous solutions.Thus, the process of Baniel is not applicable to a lactate solution ofthe type commonly obtained from a fermentation process or from othersources. The essence of the Baniel et al. U.S. Pat. No. 4,275,234 is thediscovery that efficient back-extraction can be achieved by performingthe back-extraction at a temperature higher than that of the primaryextraction.

R. Bar and J. L. Geiner, Biotechnology Progress, 3, 109 (1987) studiedthe feasibility of extracting lactic acid from aqueous solution by meansof a long-chain trialkyl amine of low basicity, such as tridodecylamine,using various tridodecylamine solutions in n-dodecanol.

REVIEW OF THE DISCLOSURE OF U.S. SER. NO. 08/207,773

In accordance with the disclosure of U.S. Ser. No. 08/207,773, it wasreported that it is possible to separate and recover lactic acid from alactate solution at a pH in the range of 4 to 14 in a nearlyquantitative fashion, with a desirable process. More specifically, thepreferred lactic acid separation and recovery process reported includesan extraction hereinafter (sometimes referred to as the primary orforward extraction) in the presence of a water-immiscible, long-chaintrialkyl amine and carbon dioxide. The lactate solution could beobtained from a fermentation broth or from hydrolyzed polylactide viapolylactide recycling or recovery, among possible others.

In the parent disclosure, the invention is described as providing aprocess for the separation and/or recovery of lactic acid from a lactatesolution formed by fermentation in the presence of a basic substancesuch as one selected from the group of alkali metal, alkaline earthmetal or ammonium hydroxides, carbonates or bicarbonates. The processsteps comprise obtaining a lactate feed solution from a fermentationbroth or another source and combining such feed solution with anextractant. The particular preferred extractants disclosed are trialkylamines, with the extraction being in the presence of carbon dioxide andwith the trialkyl amine being water-immiscible and having a total of atleast 18 carbon atoms. The term “combining” is explained in the parentas meaning a mixing or contacting of the lactate solution (aqueousphase) and the amine (organic phase) so that extraction can occur. Itwas recited that preferably the lactate feed solution is formed byfiltering a fermentation broth to remove biomass and other solids andthat the combining of the lactate solution and extractants preferablyoccurs in the presence of carbon dioxide at a partial pressure of atleast about 50 psig.

The above extraction in accordance with the parent disclosure results inthe formation of a lactic acid rich organic phase and an aqueous oraqueous-slurry phase. Each of these two phases, in accordance withpreferred further aspects of the parent disclosure, is furtherprocessed: the processing being recovery of lactic acid from the organicphase and recovery of carbonate or bicarbonate from the aqueous phase.As explained in the parent, preferably the recovered carbonate orbicarbonate is recycled to the fermentor. The organic phase from whichthe lactic acid been recovered can be recycled for use in the primaryextraction. If applied in the preferred manners described, this wouldresult in a process in which the consumption of acids and bases isavoided and in which the generation of waste salts and other by-productsis substantially reduced, if not eliminated.

In a preferred process of the parent disclosure, a countercurrentliquid—liquid extractor or extraction unit is used. During steady stateoperation, the lactate feed solution and extractant are loaded into theextractor and operated in the presence of pressurized carbon dioxide.The optimum operational pressure was described as not being critical,provided sufficient carbon dioxide is present for the primary extractionto occur. It was stated, however, that preferably the partial pressureof carbon dioxide is maintained at 50 psig or greater. It was describedthat upon leaving the extractor, the organic phase may be subjected todecompression. This would result in a release of the pressurized carbondioxide which could, if desired, be recovered for reuse in the process.

The long-chain trialkyl amines described in the parent as useful andpreferred are those in which the amines and the amine lactate salts areimmiscible with water and have a total of at least 18 carbon atoms, andpreferably have from 24 to 42 carbon atoms. Typical examples of suchamines provided in the parent are trihexylamine, trioctylamine,triisooctylamine, tricaprylylamine and tridodecylamine. The parentdisclosure states that the term “amine salt” or “amine lactate salt”refers to the species formed when lactic acid or lactate is extractedinto the amine extractant phase, although the exact nature of thisspecies is not known.

In the parent, the extraction process is described as being performedbatchwise or continuously, but that dramatically improved separation andultimate recovery can be achieved with a continuous process and inparticular a countercurrent extraction process.

In the parent it is stated that solvents of the trialkyl amines may alsobe used, if desired, as part of the extractant. It is believed thatthese may be used for the purpose of diluting certain relatively viscoustrialkyl amines, enhancing the extraction, and/or stabilizing andmaintaining the organic phase in a single phase substantially immisciblewith water. Any compatible organic solvent capable of dissolving theamine and the amine lactate salt would be suitable, provided it is alsoinert to chemical reaction both with the long-chain trialkyl aminesutilized and to the amine lactate salt and lactic acid. In the parent,it was stated that the term “compatible” means miscible with, soluble inand chemically inert. The usefulness of solvents for these purposes iswell known in the art. Specific examples, however, were described in theparent as liquid hydrocarbons such as kerosene or mineral oils, alkanolssuch as isopropanol, n-butanol and n-octanol and various ketones such asmethyl-isobutyl ketone (MiBK) and nonanone, among others. In the parentit was stated that two or more different solvents may be used, forexample a hydrocarbon and an alkanol.

According to the parent, the organic phase resulting from the primary orforward extraction is subjected to a separation process such as furtherextraction, vaporization or the like to recover the lactic acid. Also,it was stated that preferably the organic phase is subjected toback-extraction with water to recover the lactic acid in an aqueousphase. Where the initial extracting medium also contains an alkanol orketone as a solvent, it was stated that the back-extraction may bepreceded by removal of the solvent through azeotropic steam distillationor other techniques. It was also stated that the portion of the organicphase remaining after separation of the lactic acid and, whereapplicable, the separately recovered alkanol or ketone, can be recycledfor use in the primary extraction. The aqueous lactic acid solutionresulting from the back-extraction can be removed as product and can beconcentrated, if desired.

In a preferred embodiment of the process described in the parent,carbonate or bicarbonate is present in the aqueous phase either insolution or as a solid suspension, predominantly in the form of analkali metal, alkaline earth metal or ammonium carbonate or bicarbonate,depending on the cation present in the lactate solution. This aqueousphase is preferably a suspension of sodium bicarbonate crystals and issubjected to solid-liquid separation followed by conversion of thebicarbonate into sodium carbonate by heat treatment or other techniquesknown in the art. Carbon dioxide liberated during this conversion may betrapped and recycled for use in the primary extraction. The solid-liquidseparation also yields an aqueous raffinate, substantially depleted oflactate, which is withdrawn and may be used as a constituent of animalfeed.

SUMMARY

In this section, a general discussion of the principles presented in theparent disclosure 08/207,773 and its parent 08/084,810, is provided.This section includes some additional comment on those principles,beyond the specific language of the '773 and '810 applications.

In the Background section of the parent disclosure, a technique forlactic acid recovery involving acidification of a lactate solutionincluding calcium lactate, by sulfuric acid, was described. In general,it was stated that after the fermentation broth was acidified withsulfuric acid, using this prior art technique, and the mother liquor isconcentrated, the lactic acid is extracted with an organic extractant.It was generally shown that next the organic phase (water-immisciblephase) can be back-extracted with water (or the acid is adsorbed on asuitable absorbent) for isolation and recovery of the lactic acid. Thismethod is a general method involving the acidification of the broth withstrong acids. A strong acid can substantially lower the pH of thelactate solution, causing formation of undissociated lactic acid (i.e.nolactate but essentially only lactic acid) and a salt of the strong acid.In the technique, an extractant is then used to extract theundissociated acid. A principal problem with this approach is thegeneration of large amounts of the salt of the strong acid used in theacidification process. In addition, the salt of the strong acid istypically too weak a base to be useful as a base in the fermentationbroth to provide lactate salt.

Also in the Background of the parent, the Yates '323 reference wasdiscussed. It is noted that one of the carboxylic acids isolated duringthat process, is acetic acid. Acetic acid is a relatively weak organicacid. If a stronger organic acid were used in the process, it wouldtypically be even more dissociated at a given or operating pH for theextraction, than was the acetic acid, unless a very basic operating pHwere involved, i.e. one that ensured essentially all of the acid weredissociated. If the Yates' process (including extraction with a liquidpolar organic solvent as the extractant) were used, one would expecteven lower recovery of a stronger organic acid than Yates achieved foracetic acid, under the conditions of the process, since the processconcerns recovery of the acid, not the anion or base form. The recoveryrates reported in Yates '323 for acetic acid (4.8%-18%) were alreadyrelatively low. Thus, the Yates' approach would not be expected to bevery fruitful, if an acid substantially stronger than acetic acid, forexample lactic acid, were involved.

The techniques developed and reported in the parent disclosures (i.e.Ser. Nos. 08/207,773 and 08/084,810) concern and take advantage of thefact that lactic acid is moderate acid, not an extremely strong one andnot an extremely weak one. For purposes of the description, generally a“strong acid” will be considered to be any acid with a pK_(a) of 1.0 orless. A “moderate acid” would be one having a pK_(a) greater than 1.0and no greater than about 4.0. “Weak acids” would be those with a pK_(a)greater than about 4.0. For purposes of classifying an acid according tothis description, the PK_(a) should generally be rounded to the nearest0.1. In general, lactic acid, with a pK_(a) of about 3.9, then, will beconsidered a moderate acid for purposes of this discussion. The pK_(a)values in this paragraph refer to values at 25° C.; i.e. roomtemperature.

In the parent disclosures, reference was made to conducting extractionswith immiscible amines such as long chain trialkyl amines. These aretypically moderate bases. In general a “weak” base will be consideredherein to be a base with a pH of half neutralization of less than 2.5; amoderate base will be considered to be a base with a pH of halfneutralization of between 2.5 and 7.0; and, a strong base will beconsidered to be a base with a pH of half neutralization of greater than7.0. The term “pH of half neutralization” is a measure of apparentbasicity of a water-immiscible base, as defined in Grinstead, R. R. etal., J. Phys. Chem., Vol. 72 #5, p. 1630 (1968), incorporated herein byreference.

In general, the invention concerns a process for recovery of lacticacid. The process includes the steps of forming a multi-phase systemincluding at least a first aqueous phase and a second organic orwater-immiscible phase. The first aqueous phase is provided in a formhaving a pH of 4 to 14 and including lactate salt in solution. Theorganic or water-immiscible phase preferably includes an extractantcapable of forming a water-immiscible lactate salt, with alactate-containing component.

The process includes a step of extracting a first aqueous phase with awater-immiscible phase by forming a water-immiscible lactate salt withthe extractant. The step of forming the lactate salt generally includesa step of acidifying at least one of the first aqueous phase and thewater-immiscible phase without providing the first aqueous phase with apH below 4. The process further includes a step of separating theresulting water-immiscible phase from a resulting aqueous phase, afterthe step of extracting; and, obtaining or generating lactic acid fromthe lactate salt of the extractant found in the water-immiscible phase.This later step is typically conducted through a form ofback-extracting. The step of acidifying is preferably conducted byadding an acid to either or both of the first aqueous phase and thesecond water-immiscible phase. It may be conducted either before thesetwo phases are brought together, or after.

Herein reference will, in some instances, be made to “lactate-containingcomponent” in the aqueous phase. This term is intended to refer to boththe anion form, lactate anion; and, the acid form, lactic acid,together. Preferred processes result in extraction of at least 90% ofthe lactate-containing component in the first aqueous phase, andgenerally and preferably are conducted until at least 95% of thismaterial is extracted into the water-immiscible phase (as lactic acid).Indeed, preferably the process is conducted such that ultimately suchamounts (i.e. at least 90%, and typically 95% or greater) are found inthe final yield after the step of lactic acid recovery from thewater-immiscible phase.

In general, the lactic acid separation and recovery process of theinvention concerns a step of extracting the lactate-containing componentwith a water-immiscible weak or moderate base. However, it is preferredthat the lactate-containing component removed be removed after theacidifying by addition of a moderate to weak acid. The particularpreferred acid disclosed in the parent, is gaseous CO₂, a weak acidwhich will generate carbonic acid in the solution. The carbonic acid,being a weak acid, will partly dissociate in the solution. Preferablythe CO₂ is added to the water-immiscible phase, before the multi-phasesystem is formed.

The use of the weak or moderate base for conduct of the extraction isimportant, since it facilitates later recovery of the lactic acid fromthe organic phase or layer. In particular, especially if aback-extraction with an aqueous phase or layer is used for finalisolation of the lactic acid, the weak or moderate base will not holdthe lactic acid sufficiently strongly to resist the partitioning of thelactic acid back into the aqueous phase. Preferably the back-extractionis not conducted with a weak base alone. Preferably it is conducted witha moderate base, or a mixture of weak base and moderate base.

The use of a relatively weak acid to acidify, prior to extraction orduring extraction, is also important. The relatively weak acid will forma salt in the aqueous phase which is a relatively strong base that canbe used directly, or after mild conversion (for example from abicarbonate to a carbonate), as a base in the fermentation broth. Theuse of a moderate acid is less preferable because in general the saltformed will be too weak a base to be readily useful as a base in thefermentation broth.

In general, the weak or moderate base, used for the extraction, will beconsidered immiscible if, under the extraction conditions, it issufficiently insoluble in the aqueous phase or layer such that itspresence will be no greater than about 1000 ppm, and preferred ones willhave a presence of no greater than about 200 ppm. Typically, bases willbe chosen that have a solubility of 100 ppm or less. The weak ormoderate base could be presented in the form of a solid, and thus becompletely immiscible in water.

Also, preferably the weak or moderate base is also immiscible in thewater used in the later back extraction, if one is conducted. A basewill be considered immiscible in the aqueous phase of the backextraction if the above-stated ppm limits for water-immiscibility duringthe first extraction are not exceeded.

From the above it will be apparent that the process steps of preferredlactic acid recovery processes, described in the parent applications,concern the following steps:

(1) Obtaining a lactate feed solution from a fermentation broth or othersource;

(2) Modifying the lactate feed solution, preferably with a source ofmoderate or weak acid while maintaining a pH of at least 4, andpreferably within the range of 4-14; and

(3) Extracting the lactic acid with a water-immiscible weak or moderatebase, or mixture.

In the preferred applications described, the water-immiscible weak ormoderate base is a water-immiscible amine, typically an alkyl amine andpreferably a water-immiscible tertiary amine. Preferably alkyl aminesand most preferably water-immiscible trialkyl amines are used. Asexplained in the parent, the preferred water-immiscible amine will be atrialkyl amine containing at least a total of 18 carbon atoms.

In an alternate statement, typically when the extraction occurs, theaqueous phase is provided such that the lactate-containing component orspecies present, at any given time throughout the extraction, is atleast 50% (molar equivalent) in the form of lactate, rather than thelactic acid. In general this is accomplished by appropriate control ofthe pH, and selection of the desirable acid for acidification of thesystem.

The preferred processes are conducted on lactate feed solutions thatcontain lactate values in a concentration of at least 3 mol. When theprocesses concern recovery from a fermentation broth, generally thefermentation process is conducted such that the feed solution containsat least 5%, and typically 10 to 30%, by weight, lactate. The broth maybe concentrated, before extraction.

As also will be apparent from the parent disclosures, preferably themoderate or weak acid, used to acidify, is an acid which is eitherreadily separated from the water-immiscible weak or moderate base, orwhich does not combine to any substantial extent with thewater-immiscible weak or moderate base under the conditions ofextraction. Carbon dioxide is an almost ideal acid for use in generationof the lactic acid, since its presence can so readily be controlledthrough control of its partial pressure, it can easily be removed fromthe solutions, it is relatively inexpensive, and it is such a weak acidthat the salt which is generated is also very suitable for effectiveneutralization of fermentation broth. It is foreseen that in someinstances the moderate or weak acid may comprise a salt of an acidhaving more than one proton; for example monosodium phosphate, providedthe pK_(a) for the remaining proton(s) is within the appropriate ranges.

In typical preferred applications, the acid which is used to acidify themulti-phase system is preferably a weaker acid (i.e. has a higherpK_(a)) than lactic acid. A reason for this preference is that thecorresponding salt of the added acid, which will form in the aqueousphase, will be a useful base in the fermentation broth. Also, when usedwith the preferred extractants, such as the water-immiscible amines, anadvantage results because of the extractant's preference for thestronger acid, i.e. lactic acid.

As indicated in the experiments, processes according to the inventionare characterized by relatively high recoveries of the lactate values inthe lactate containing feed solution (for example, the fermentationbroth). Recoveries greater than 90% are readily achieved, and typicallythe recovery is at about 95% or greater.

With respect to the weak or moderate base, to be used in the extraction,as indicated in the parent application, it has been found that trialkylamines having a total of at least 18 carbon atoms, and preferably from24-42 carbon atoms, are most desired. Among the ones identified in theparent application, tridodecylamine presently appears preferred.However, in general it is believed that while tertiary amines arepreferred, substituted tertiary amines and in some instances evenprimary or secondary amines may also be used, provided they aresufficiently water-immiscible and perform as weak or moderate bases. Itis foreseen that, in many instances, primary amines may have sufficientwater solubility to be undesirable with respect to possiblecontamination of the aqueous phase. It is also foreseen, that in someinstances, primary or secondary amines may have too great a propensityto react with the lactic acid to form an amide, to be fully preferred.However, there is no theoretical reason why at least some secondary orprimary amines could not be used, under certain circumstances.

Although the general principles have been described with respect toconduct of the extraction with either a weak base or a moderate base (ormixture of both), in general it is believed that if a weak base is usedalone, the extraction results will not be as great as preferred becausethe weak base typically will prefer the undissociated acid, which wasthe weaker acid used for acidulation. The lactic acid, which is largelydissociated, is thus less effectively extracted by a weak base (when theweaker acid is used). Thus it is foreseen that if a weak base is to beused, it will generally be preferred to use it in a mixture with amoderate base as well. A weak base is typically used as a co-solvent (orsolvent) in the water-immiscible phase, with a moderate base alsopresent to facilitate extraction.

In the parent application, it was explained that while a variety ofextraction processes may be used, it was foreseen that countercurrentextraction processes would be preferred. In addition to providing forcountercurrent contacting of the two liquid phases, preferably theextraction unit used should provide for good removal and flow of thesolids formed during precipitation of the salt of the moderate to weakacid. In the preferred processes described, this would be thebicarbonate salt of the cation in the fermentation broth. Typically thecation will be Na⁺. In the most preferred embodiments, wherein thelactate feed treated and extracted has a greater than 3 mol. lactateconcentration and the extraction is conducted with a water-immiscibletrialkyl amine, the aqueous phase is relatively small in volume and, mayfor a slurry with the solid precipitate.

In the parent applications, solvents and/or diluent for the trialkylamines, i.e. solvents and/or diluents for the water-immiscible bases,were described. In general, the selection of solvent will be controlledby a variety of factors such as boiling point, water solubility, andability to enhance the extraction efficiency of the water-immisciblemoderate base. However, in general when the lactic acid recovery is froma lactate containing fermentation broth, it is desired that theextraction occur within as short a period of time of the generation ofthe fermentation broth as possible, for commercial efficiencies. Mostfermentation broths are separated from the fermentation process at atemperature of about 50° C., in typical commercial operations. Thus,unless time is used in substantial cooling, or cooling equipment isused, the extraction will typically occur of an aqueous solution havinga temperature of at last 35° C., usually about 40-50° C.

If a back-extraction is used to recover the lactic acid from the organicphase, preferably the aqueous phase used during the back-extraction isat a higher temperature than the aqueous phase from which the lacticacid is separated in the first instance. This helps increase recoveryefficiencies. Preferably the temperature of the back-extraction is ashigh as reasonably possible. Indeed, preferably it is conducted at atemperature of at least 100° C., and typically and preferably at least135° C.

When a solvent and/or diluent is present in the organic phase, as willgenerally be preferred, the extraction is preferably conducted below theboiling point of the solvent and/or diluent. Thus, the temperature ofthe desired aqueous phase will, in some instances, dictate the preferredsolvent used. It has been found that a mixture of paraffin (Isopar K,from Exxon) and octanol is a desirable solvent for the organic phase,and the use of octanol allows extraction temperatures up to about140°-160° C. The mixture would preferably comprise about 50% trialkylamine, 30% n-octanol and 20% non-aromatic paraffin, by weight.

It is noted that in some instances the solvent (or co-solvent) in thewater-immiscible phase will function as a weak base and thus be capableof some modest amount of extraction. This would be true, for example,when the solvent is an alcohol or a ketone. However, alcohols andketones are generally such weak bases that their operation in extractingthe lactic acid is more akin to a salvation process than a more tightlyassociated ion attraction with the lactic acid. Thus the presence of atleast some moderate base, such as the amine bases, is generallypreferred. The presence of a polar solvent such as an alcohol or ketonecan enhance the ability of a moderate base, such as a water-immiscibleamine, to extract an acid, such as lactic acid.

If the organic phase is back-extracted, for final recovery of the lacticacid, in some instances it may be preferable to conduct theback-extraction under carbon dioxide pressure, in order to facilitatethe extraction. For example, if the primary extraction was conductedunder carbon dioxide pressure, the conduct of the back-extraction underpressure will prevent CO₂ release or the need to repressurize theescaping CO₂.

In preferred applications, the step of acidifying includes acidifyingthe aqueous phase from a fermentation process, to form sodiumbicarbonate in the aqueous phase; and, the process includes using thesodium bicarbonate to form sodium lactate in the first aqueous phase, inlater processing. This latter step may include forming sodium carbonatefrom the sodium bicarbonate and then putting the sodium carbonate in afermentation process.

DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a block diagram representing thepreferred embodiment of the process according to the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PROCESS

With reference to the drawing, lactic acid fermentation is carried outin a fermentor 10 in which carbohydrates are fermented and converted tolactic acid by the bacterial genus Lactobacillus and more specificallyby the microorganism Lactobacillus acidophilus. Because many organismswhich are attractive in such a fermentation process cannot tolerateacidic conditions with a pH lower than about 3.8, the acids formed bythis process must be at least partly neutralized to maintain the pHabove such level and more preferably above a pH of 4.5 to allow thefermentation to continue. In accordance with the preferred process, aneutralizing agent such as the alkali metal, alkaline earth metal orammonium hydroxides, carbonates or bicarbonates are used for thispurpose. In the preferred process, sodium carbonate (Na₂CO₃) is added.to the fermentor 10 for this purpose, either via the recycle 12 asdiscussed below or along the path 13. Preferably, sufficient sodiumcarbonate or other alkaline substance is provided to the fermentor 10 tomaintain the pH of the fermentation broth at a pH above 5.0 andpreferably in the range of about 5.0 to 7.0, more preferably in therange of about 5.5 to 6.5 and most preferably in the range of about 6.0to 6.5. Other ingredients may also be used in the fermentation processwhich is well known in the art.

In the fermentor 10, the carbohydrate is converted to lactic acid whichimmediately is converted to a lactate form in the presence of theneutralizing agent. In the preferred process using sodium carbonate,sodium lactate [NaCH₃CH(OH)COO] is formed. A portion of the fermentationbroth or liquor is continuously or intermittently withdrawn from thefermentor 10 via the path 14 and exposed to a filtration andconcentration unit 15. The unit 15 functions to physically remove, viafiltration or ultrafiltration, biomass and other solids which can berecycled to the fermentor 10, if desired. The filtrate comprises anaqueous lactate solution which contains the lactate salt comprised ofthe lactate anion together with the cation of the neutralizing agent. Inthe preferred process, the filtrate is comprised principally of sodiumlactate. This solution, which commonly comprises between about 0.25% and50% by weight of sodium lactate, may be concentrated by waterevaporation or other techniques to improve the overall lactic acidproduction efficiency. In the preferred process, the filtered lactatesolution is concentrated by water evaporation to about 40% to 70% byweight sodium lactate; however, such concentration is optional.

The sodium lactate solution exiting from the filtration andconcentration unit 15 comprises a lactate feed solution which is fedinto an extraction unit 18 along the path 16. The unit 18 is part of anextraction system which also includes the extractant regeneration unit22, the organic phase stream 21 and the extractant recycle stream 24.Within the unit 18, the lactate feed solution is combined with anextractant comprised of at least one water-immiscible trialkyl amine inthe presence of carbon dioxide, where the amine has a total of at least18 carbon atoms. Within the unit 18, two separate phases are formed: anorganic phase containing the extractant and extracted lactic acid and anaqueous or aqueous-slurry phase containing the carbonate or bicarbonatesalt of the cation of the neutralizing agent. In the preferred process,the aqueous phase contains sodium carbonate or bicarbonate. The unit 18may comprise any one of a variety of single or multi-stage pressureextraction units. In the preferred process, the unit 18 is a multi-stagecountercurrent extraction unit.

In the preferred process, the extraction system is initially chargedwith the trialkyl amine. The amine may be introduced by directly addingit to the unit 18 or by adding it to the recycle stream 24 through theamine make-up stream 17. During steady state operation, little if anyadditional trialkyl amine will be needed. To the extent it is, however,it can be added to the recycle stream 24 via the make-up stream 17.

Carbon dioxide may be added directly to the unit 18 under pressure viathe page 19 a, to the organic recycle stream 24 under pressure via thepath 19 c, or to the aqueous lactate stream 16 under pressure via thepath 19 b. In the preferred process as illustrated, the organic recyclestream 24 is preloaded with carbon dioxide by adding carbon dioxideunder pressure via the stream 19 c prior to the unit 18. In any case,the carbon dioxide within the unit 18 should preferably be maintained ata partial pressure of at least about 50 psig, more preferably at apartial pressure of at least 75 psig and most preferably between about150-500 psig.

Although it is believed that some extraction of lactic acid from alactate solution is possible with any water-immiscible trialkyl amine inthe presence of carbon dioxide, the particular degree of extraction willvary with the amine utilized and the carbon dioxide pressure. The degreeof extraction can also be enhanced or otherwise affected by varioussolvents as described below and as known in the art. The degree ofextraction will generally be dependent on the partition coefficient andthe number of stages used in the extraction process. As used herein, thepartition coefficient is the mass concentration of lactic anion in theorganic phase divided by the mass concentration of lactate expressed aslactic acid equivalent in the aqueous phase. Usually, for a particularsystem, the partition coefficient, within limits, will vary directlywith the carbon dioxide pressure. As the partition coefficientincreases, the number of stages needed to achieve a particular degree ofextraction will decrease. Carbon dioxide and amine composition shouldpreferably be sufficiently high to avoid excessive extractant phasenecessary to extract the acid.

The trialkyl amines which are useful in the process of the presentinvention are those which are water-immiscible and relatively weak.Specifically, these are the trialkyl amines having a total of at least18 carbon atoms and preferably about 24 to 42 carbon atoms. Thepractical lower limit of the number of carbon atoms is limited by theincreasing water solubility of the smaller trialkyl amines or theirsalts. The water immiscibility of the trialkyl amines with 18 or morecarbon atoms is well known in the art. The practical upper limit of thenumber of amine carbon atoms is determined by the molar concentration ofamine obtainable in the organic phase.

Specifically, the extraction ability of the trialkyl amines is dependenton the molar concentration of the amine component. Thus, as themolecular weight of the amine increases, the molar concentration of theamine component (or a pure amine solution) will decrease. The trialkylamine should also be sufficiently strong to extract the lactic acid fromthe aqueous lactate feed, but sufficiently weak to enable water to backextract the lactic acid from the organic phase. Typical examples of suchamines which meet the above requirements, are readily available and areuseful in the process of the present invention are one or more oftrihexylamine, trihexylamine, trioctylamine, triisooctylamine,tricaprylylamine, tridodecylamine and mixtures thereof.

The particular ratios of lactate feed solution and trialkyl amine phasewhich are fed to the unit 18 along the paths 16 and 24, respectively,will depend on a variety of factors including the concentration of thesodium lactate and the concentration of the amine. Preferably, theintroduction of these materials should be such as to result in asubstantially complete extraction of lactic acid from the lactatesolution with the number of stages utilized. More preferably, the feedration of amine phase to lactate solution should be about 40:1 to 1:2and most preferably about 15:1 to 1:1 by weight.

The trialkyl amine provided to the unit 18 may be introduced in asubstantially pure or a diluted form. Because many of the aminesapplicable to the present process and their salts are relativelyviscous, it is preferably to introduce such amines with a solvent. Ingeneral, any composition which is miscible with the subject amines andtheir salts within the range of compositions used and which isreactively inert relative to the system components may be used in thepresent process. These solvents may be used to control viscosity,enhance extraction or stabilize the organic phase in a manner generallyknown in the art. Typical examples of solvents which can be used in thepresent process include liquid hydrocarbons such as kerosene or mineraloil, alkanols such as isopropanol, n-butanol and n-octanol and variousketones such as methyl-isobutyl ketone (MiBK) and nonanone, amongothers. The extractant used in the process of the present invention maycomprise 100% of the trialkyl amine. A more preferred extractant,however, comprises up to about 70% by weight of a solvent or shouldcomprise about 30%-95% by weight of the amine and about 5%-70% by weightof the solvent.

Following extraction within the unit 18, a lactic acid-rich organicphase comprised of lactic acid and the extractant is withdrawn along thepath 21 and an aqueous phase or slurry comprises principally ofcarbonate and/or bicarbonate is withdrawn along the path 20. As used inthe description of the preferred embodiment, the term carbonate orbicarbonate refers to the carbonate or bicarbonate salt of the cation ofthe substance used to neutralize the fermentation. Within the aqueousphase or slurry of the preferred embodiment, the predominant carbonateor bicarbonate is sodium bicarbonate which exists principally as sodiumbicarbonate crystals. These are separated from the aqueous raffinate inthe solid-liquid separation unit 25. The unit 25 can comprise variousfiltration, centrifugation or other solid-liquid separation means knownin the art. Preferably, however, the sodium bicarbonate crystals areseparated by filtration. The aqueous filtrate which in the preferredprocess is substantially free of lactate may be removed as a componentof animal feed or as waste along the path 26. It is also possible, ifdesired, to recycle all or part of the filtrate back into the systemthrough the streams 11, 14 or 16.

The separated sodium bicarbonate is then directed along the path 29 to aconversion and purification unit 30 for conversion of the sodiumbicarbonate to sodium carbonate. Means are known in the art foraccomplishing this conversion. In the preferred process, however, thesodium bicarbonate crystals are decomposed in boiling water to producecarbon dioxide and dissolved sodium carbonate.

The solution of sodium carbonate is then purified by active carbontreatment and recycled along the path 12 as an alkaline orneutralization component in the fermentation process. The releasedcarbon dioxide can also be reused, if desired. Since the preferredprocess utilizes sodium carbonate as the neutralizing component in thefermentation process, the aqueous phase after fermentation (stream 14)is comprised of sodium lactate. It is contemplated that other alkalimetals, alkaline earth metals or ammonium hydroxides, carbonates orbicarbonates may also be used as the neutralizing agent, in which eventthe cations in the aqueous phase would be altered accordingly.

The lactic acid-rich organic phase is withdrawn from the unit 18 alongthe path 21. In the preferred process, the organic phase is decompressedin the flash unit 32 as it leaves the unit 18. This results in therelease of a majority of the dissolved carbon dioxide via the stream 33which may be recycled to streams 19 a, 19 b or 19 c, if desired. Thisorganic phase is made up principally of lactic acid and the extractant.Lactic acid is separated or recovered from this phase in the extractionunit 22, leaving a lactic acid-lean or depleted organic phase which ispreferably recycled back to the extraction unit 18 along the path 24 inthe form of regenerated extractant. As described above, carbon dioxidemay also added to the recycle stream 24 via the path 19 c to load theamine prior to the unit 18. The lactic acid solution is removed from theunit 22 as product via the stream 28.

In the preferred process, the unit 22 is a liquid/liquid extraction unitwithin which the lactic acid-rich organic phase is back extracted withwater introduced along the path 23. Because of the relatively weak aminebeing used in the primary extraction process and because the amine iswater-immiscible, the water is able to extract the lactic acid from theamine to form an aqueous solution of lactic acid of acceptableconcentration.

In the case where the trialkyl amine is diluted with an appropriatesolvent, such solvent becomes a part of the organic phase withdrawn fromthe unit 18 along the pat 21. Some solvents, such as alkanols andketones, modify and enhance the lactic acid uptake into the organicphase. It is preferable to remove such solvents prior to the backextraction with water in the unit 22. This separation of the solventfrom the lactic acid-rich organic phase can be accomplished by variousseparation techniques known in the art. Preferably, when possible, theseparation is by azeotropic steam distillation within the unit 27. Theremoved solvent from the separation unit 27 may then be recycled alongthe pat 31 for regeneration of the extractant and use in the primaryextraction, if desire.

The lactic acid can also be separated or recovered from the organicphase by vaporization or distillation of the lactic acid. Removal bydistillation should preferably be performed at reduced pressure andelevated temperature conditions. Most preferably, the separation shouldbe accomplished at pressures of from about 0.2 to 100 mm Hg and attemperatures from about 80° C. to about 240° C. If this distillationoption is employed, the trialkyl amine should have a total of at least24 carbon atoms, or be sufficiently nonvolatile to allow lactic acidfractionation from the amine by vacuum distillation.

The vaporization conditions will also remove alkanols or ketones, ifpresent, as well as other solvent components more volatile than thetrialkyl amine. These can be separately recovered and may be returned tothe depleted extractant before it is cycled back to the extraction step.The vapor of lactic acid thus formed may also be directly fed, ifdesired, to an esterification process for reaction and furtherpurification.

With the above process, lactic acid can be separated and/or removed froma lactate fermentation broth. Under optimal conditions, such separationand/or recovery can approach total recovery of the lactic acid, greaterthan 95% of that produced by fermentation. Of equal or greaterimportance is the ability of this recovery to be accomplished with thegeneration of minimal, if any, waste salt and under circumstances wheresubstantially all of the extraction, conversion and other componentsused in the process can be recycled for reuse within the process. Stillfurther, the process is significantly less energy intensive thancompeting processes and results in minimal, if any, plant emissions.

The preferred process has been described with respect to producinglactic acid from a lactate solution formed via a fermentation process.The present process is, however, application to the separation and/orrecovery of lactic acid from a lactate solution regardless of itsorigin. For example, polylactide is a biodegradable polymer producedfrom lactic acid. Polylactide can be recycled by hydrolysis of thepolymer to yield a lactate salt. The present process can then be used torecover lactic acid from the lactate salt for reuse in formation of thepolylactide polymer.

Further details of the present process are shown and described in thefollowing specific examples.

EXAMPLE 1

A lactate fermentation broth containing 10% by weight of NaCH₃CH(OH)COO(sodium lactate) was withdrawn from a fermentor in which purecarbohydrates were fermented by Lactobacillus delbrueckii in thepresence of sodium carbonate in order to maintain a pH of 5.5, all asknown in the art. The biomass and other solids were removed from thefermentation broth by filtration through ultrafiltration membranes andthen concentrated by water evaporation to 50% by weight sodium lactate.

150 g/min of this sodium lactate solution were fed to a 5-statemixer-settler battery counter-currently to 1050 g/min of regeneratedextractant comprising 48% by weight tricaprylylamine (Alamine 336™produced by Henkel), 20% by weight n-butanol and 32% by weightaromatic-free kerosene. The extraction system was kept at ambienttemperature and a 240 psig CO₂ atmosphere was maintained therein. Sodiumbicarbonate crystals formed in the aqueous phase as of the secondmixer-settler.

The aqueous phase was withdrawn and sodium bicarbonate was filtered offfrom the aqueous raffinate which was practically free of lactate values.The sodium bicarbonate crystals were decomposed in boiling water to CO₂and to dissolved sodium carbonate. This solution was purified by activecarbon treatment to a form suitable for use as a base in thefermentation.

The organic phase withdrawn from the last stage of the mixer-settlerbattery contained 0.65 mole lactic acid per kg. CO₂ was allowed toescape from the organic phase, following which the butanol was separatedand removed by azeotropic steam distillation. The remaining organicphase was back-extracted with hot water to form a lactic acid-depletedorganic phase and an aqueous solution of the lactic acid. The separatedbutanol was reintroduced into the back-extracted organic phase toregenerate the extractant while the aqueous phase was concentrated andfed to final purification. Removal of lactic acid from the lactate feedsolution was about 95%.

EXAMPLE 2

Aqueous solutions of sodium lactate were equilibrated in a pressurevessel with various extractants under a 220 psig CO₂ atmosphere. Contacttemperature was 20° C. The initial pH of the aqueous phase andequilibrium data are summarized in the Table below.

TABLE 1 Equilibrium data sodium lactic Initial lactate acid Extractantaqueous aqueous organic composition pH (wt %) (wt %) n-butanol 5.5 6.21.05 80% TDA + 20% i-PrOH 5.5 29.2 10.6 80% TDA + 20% hexane 5.5 39.82.9 67% TCA + 33% n-OctOH 10.9 50.3 14.3 70% TCA + 30% n-BUOH 10.9 50.315.0 48% TCA + 20% n-BUOH + 10.7 46.9 9.3 32% kerosene TDA =tridodecylamine (Henkel) TCA = tricaprylylamine i-ProOH = isopropanoln-OctOH = n-octanol n-BuOH = n-butanol

EXAMPLE 3

An extractant mixture comprised of 80% by weight tridodecylamine(Alamine 304-1) produced by Henkel) and 20% by weight n-butanol, wascontacted with 30% by weight aqueous lactic acid (Purac) in sufficientquantity to produce a loading of 6.9% by weight lactic acid in theorganic phase. 230 g of this material was added to a stirred roundbottom flask connected to a distillation apparatus, condenser, andcontrolled vacuum system. The solution was heated to 219° C. at apressure of 2 mm Hg. Initial condensate fractions included butanol andwater. A later fraction showed recovery of 97% by weight of the originallactic acid. The residual extractant contained 0.2% by weight lacticacid. The composition of the pooled fractions containing the acid was98.3% by weight aqueous lactic acid and 1.7% by weight of its oligomers.The depleted extractant was replenished with butanol and cycled back foranother extraction. Five such cycles were rn on one batch withoutsignificant loss of extractant performance.

EXAMPLE 4

Various experiments were conducted in a Parr pressure reactor for thepurpose of showing the applicability of the process of the presentinvention to a broad range of CO₂ pressures, to a variety of solventsand to representative samples of trialkyl water-immiscible amines with atotal carbon content of at least 18.

The apparatus comprised a Parr pressure reactor with four agitators, gasinlet and outlet ports, aqueous and organic sample ports and a pressuregage. The procedure involved adding the aqueous and the pre-mixedorganic solutions to the Parr reactor. Except for Experiment Nos. 7, 12,18 and 21 below in which the ratio of organic to aqueous was 1:3, theratio of organic to aqueous in all experiments was 1:1. The aqueoussolution was comprised of sodium lactate (NaLa), calcium lactate (CaLa)or potassium lactate (KLa). The NaLa solutions comprised about 20%-40%by weight of the lactate, the KLa solution comprised about 20% by weightof the lactate, while the CaLa solution comprised about 6% by weight ofthe lactate. Further, in Experiment Nos. 7-10, 12-18 and 20-24, 10% byweight sodium bicarbonate was added for the purpose of saturating thesolution. The organic solution comprised a trialkyl amine or a mixtureof a trialkyl amine and one or more solvents.

The Parr pressure reactor was then assembled and a slow flow of CO₂ wasintroduced for about 5 minutes to purge the air in the reactor. The CO₂pressure was then adjusted to the desired level. It should be noted thatthe pressures identified in the table below are gage pressures. Thus, aCO₂ level of 0 psig as indicated in Experiment Nos. 1 and 13 reflect aCO₂ partial pressure of 14.7 psi. The CO₂ pressure was maintained at theselected level, with a slow bleed of CO₂ (about 100 ml/min) bubblingthrough the contents, and the contents in the reactor were agitated fortwo hours. The CO₂ inlet and outlet ports were then sealed and agitationcontinued for 10 more minutes, at which time agitation was terminatedand the contents were allowed to settle for 30 minutes. Samples of boththe organic and aqueous phases were collected through the organic andaqueous sample ports, after which the above procedure repeated for adifferent CO₂ pressure. All experiments were run at 25° C.

All organic samples were analyzed with NaOH to a phenolphthaleinendpoint to determine concentration of lactic acid in the organic phase.All aqueous samples were analyzed by HPLC to determine lactic acidequivalent in the aqueous phase. The partition coefficient (K) was thencalculated by dividing the concentration of lactic acid in the organicphase by the concentration of lactate, expressed by lactic acidequivalent, in the aqueous phase.

The table below reflects data from selected experiments conducted inaccordance with the above procedure in which the amines and solvents areidentified as follows. All percentages are by weight unless otherwisespecified.

Amines Solvents A1 = trihexylamine CS1 = n-octanol A2 = trioctylamineCS2 = n-butanol A3 = triisooctylamine CS3 = nonanone A4 =tricaprylylamine CS4 = Isopar K, Exxon A5 = tridodecylamine

TABLE 2 CO₂ Press K Final Aq. No Organic Aqueous (psig) C(org)/C(ag) Wt% Lac.  1 A4(48%), CS1(30%), CS4(22%) CaLa 0 0.068 2.21  2 A4(48%),CS1(30%), CS4(22%) CaLa 150 0.388 2.06  3 A4(48%), CS1(30%), CS4(22%)CaLa 220 0.553 1.99  4 A4(48%), CS1(30%), CS4(22%) CaLa 300 0.622 1.93 5 A4(48%), CS1(30%), CS4(22%) KLa 75 0.112 18.5  6 A4(48%), CS1(30%),CS4(22%) KLa 500 0.198 17.5  7 A4(48%), CS1(30%), CS4(22%) NaLa(sat) 750.047 19.9  8 A4(48%), CS1(40%), CS4(12%) NaLa(sat) 150 0.093 38.3  9A4(48%), CS1(40%), CS4(12%) NaLa(sat) 200 0.155 38.0 10 A4(48%),CS1(40%), CS4(12%) NaLa(sat) 300 0.199 37.5 11 A4(48%), CS2(35%),CS4(17%) NaLa 220 0.255 29.8 12 A1(33%), CS1(30%), CS4(37%) NaLa(sat)240 0.123 20.1 13 A2(43%), CS1(30%), CS4(27%) NaLa(sat) 0 0.038 21.9 14A2(43%), CS1(30%), CS4(27%) NaLa(sat) 75 0.064 21.6 15 A2(43%),CS1(30%), CS4(27%) NaLa(sat) 240 0.119 20.7 16 A4(48%), CS3(52%)NaLa(sat) 240 0.069 21.6 17 A3(43%), CS1(30%), CS4(27%) NaLa(sat) 750.038 20.3 18 A3(43%), CS1(30%), CS4(27%) NaLa(sat) 240 0.097 20.0 19AS(48%), CS2(20%), CS4(32%) NaLa 220 0.266 34.4 20 A4(100%) NaLa(sat) 750.020 21.4 21 A4(100%) NaLa(sat) 240 0.047 21.4 22 A4(43%),CS1(30%),CS4(27%) NaLa(sat) 500 0.208 19.8 23 A4(48%), CS3(52%)NaLa(sat) 500 0.102 21.3 24 A4(48%), CS4(52%) NaLa(sat) 500 0.020 21.8

EXAMPLE 5 Extraction of Lactate Feeds at Various Conditions

Sodium and potassium lactate solutions were prepared stoichiometricallyfrom lactic acid solutions. A304 is Alamine 304, Trilaurylamine fromHenkel Corp. The concentration was adjusted to 1 mol/kg by diluting withParasol, a non-aromatic kerosine solvent, boiling range 210-275° C.,from Paz Company of Israel. The alcohols were supplied by Merck orFrutarom of Israel.

The pressure experiments were carried out in a Parr bench-top minireactor (Serial 4560, 300 ml autoclave). CO₂ was supplied via acylinder. The control panel regulated the heating and measured internalpressure in psi. Stirrer speed was 600 rpm. At each pressure level astable pressure was held for 15-20 minutes to ensure completeequilibrium and then phase separation for a further 15-20 minutes.Samples were taken (for Table 3 experiments) from the organic phase,which were usually clear. Free CO₂ volume was measured by waterdisplacement. Lactic acid concentration was determined by titration with0.1 N NaOH and phenolphthalein indicator after warming the sample forremoval of extracted CO₂.

TABLE 3 CO_(2/) Loading gm H₂O Org. Org. Phase Org. Conversion/ InitialPressure Temp Phase Phase Ratio Phase Extraction Exp Amine Conc. Modif.Conc. Aqueous Conc. psi ° C. mol/kg cc org/ag % % 19 A304 1 mol/kg2-BuOH 20% NaLa 40%  87 14° 0.27 3 145 0.43 212 0.539 20 None 2-BuOH100%  NaLa 40%  85 14° 0.114 3 146 0.1 218 0.113 21 A304 1 mol/kg 2-BuOH20% NaLa  5%  88 14° 0.053 3 146 0.069 217 0.082 22 A304 1 mol/kg 2-BuOH20% KLa    8.30%  87 13° 0.021 15.3 3 147 0.019 22.9 218 0.027 36.7 23A304 1 mol/kg 2-BuOH 20% NaLa 20% 218 18° 0.228 3 24 A304 1 mol/kg2-BuOH 20% NaLa 10% 218 14° 0.1 35.9 3 25 A304 1 mol/kg i-BuOH 30% NaLa56% 238 18° 0.923 73.9 3 57.4 26 A304 1 mol/kg i-BuOH 30% NaLa 56% 24018° 0.785 43.4 5 1.41 82.1 27 A304 1 mol/kg i-BuOH 30% KLa 59% 238 18°0.657 39.9 5 1.26 71.9 28 A304 1 mol/kg i-PrOH 20% NaLa 56% 244  0°0.831 90.0 5 1.46 86.1 238 16° 0.781 40.1 5 1.01 82.0 237 50° 0.464 62.55 1.44 47.7 29 A304 1 mol/kg EtOH 15% NaLa 56% 239  0° 0.655 73.6 5 1.430 A304 1 mol/kg i-PrOH 20% NaLa 56% 237  0° 0.707 62.7 5 67.7 238 17°0.859 44.0 5 83.3 31 A304 1 mol/kg EtOH 15% NaLa   51.80% 238 20° 0.72248.2 5 75.5 32 None EtOH 100%  NaLa   51.80% 236 21° 0.143 75.1 5 33A304 1 mol/kg n-Octanol 20% NaLa   51.80% 240 20° 0.526 34.1 7 75.9 34A304 1 mol/kg i-PrOH 30% NaLa   51.80% 241 18° 0.848 43.4 5 240 40°0.803 31.82 5 238 59° 0.593 31.36 5

EXAMPLE 6 Extraction with Variations in Extractant and pH

Materials

A304 Alamine 304, Trilauryl Amine. Henkel Corp. commercial product

A336 Alamine 336, Tri-Caprylyl Amine (C₈-C₁₀). Henkel Corp. commercialproduct

DEHPA Di-(2-ethylhexyl) Phosphate Sigma, Anhydrous reagent

i-AmOH Merck A.R.

1-BuOH Frutarom A.R.

2-BuOH Frutarom A.R.

i BuOH Merck A.R.

n-Octanol Merck A.R.

Lactic acid (LaH) Merck A.R., 90% solution

MIBK Methylisobutylketone Frutarom C.P.

MTCA Aliquat 336, Methyltricapryl amine chloride, Henkel Corp.

n-Butyl Acetate BDH GPR

Parasol Kerosine solvent <1% aromatics, boiling range 210-275° C., PazCompany

Primene JM-T 5-Alkyl Primary Amine (a primary aliphatic amine withhighly branched alkyl chains, nitrogen is bonded directly to a tertiarycarbon, R₁C(R₂) (R₃)NH₂) ROHM & HAAS, commercial product

TBP Tri-Butyl Phosphate Riedel-de Haen 99% solution

TOPO Trioctylphosphine oxide, Sigma 90% tech

Xylene Frutarom A.R.

Methods

Distribution curves were determined by limiting condition experimentscarried out in 100 ml erhlenmeyers. Analysis of clear organic phase forloading of lactic acid was by direct titration with NaOH 0.1N inisopropanol using phenolphthalein as indicator. The aqueous phases weresimilarly analyzed in H₂O.

Sodium lactate solutions at the various concentrations and pH's wereprepared stoichiometrically from lactic acid solutions then adjusted tothe required pH. The extraction curves for the various extractants andsolvents were made at limiting conditions and analysis as above on clearorganic phase.

For all experiments the lactic acid was diluted from the originalbottled solution (90%) and hydrolysed by refluxing for 4-6 hours.

TABLE 4 pH = pH = pH = pH = Organic Phase 2-2.3 3-3.1 4-4.1 pH = 4.96-6.3 pH =6.9 pH = 8 1 mol/kg A304 in Parasol + 1.26 1.07 0.57 0.0570.011 100% 1-BuOH 1 mol/kg A304 in Parasol + 1.48 1.48 1.07 0.29 0.011300% 1-BuOH 1 mol/kg A304 in Parasol + 1.15 0.49 0.031 0.000 20%n-octanol 1 mol/kg JMT in Parasol + 1.05 0.97 0.89 0.087 0.028 20%n-octanol 1 mol/kg JMT in n-octanol 1.15 0.82 0.089 0.048 1-butanol 0.870.25 0.000 0.000 isoamyl alcohol 0.60 0.22 0.000 0.000 TBP 0.74 0.280.016 0.000 1 mol/kg TOPO in xylene 0.75 0.68 0.56 0.21 0.043 n-butylacetate 0.10 0.032 0.000 A304:Oleic acid 0.5 mol/kg 0.35 0.100 0.0000.000 336:DEHPA 0.5 mol/kg 0.39 0.33 0.26 0.082 0.000 MTCA.DEHPA 0.5mol/kg 0.35 0.24 0.036 0.030 MTCA 1 mol/kg + 20% octanol 0.59 0.31 0.0190.000

A PROPOSED COMMERCIAL PROCESS

In the application of the disclosed invention, and based on laboratoryand pilot data, a commercial process is foreseen to have the followingpreferred configuration and conditions. The process will be describedwith reference to numerals used in the figure.

Lactate fermentation by Lactobacillus would be conducted with asubstrate (11) of dextrose, salts, and nutrients, controlled at pH 5 to7 by addition of recycled sodium carbonate (12) in standard anaerobicfermentators (10). CO₂ evolved during fermentation would be recoveredfor reuse in the extraction. After fermentation is complete, thefermentation liquor (14) would be filtered (15) to remove biomass,carbon treated, and evaporated (15) to 40-70 wt % NaLa. The cells arediscarded after the process.

The concentrated broth (16) would be pumped into the extraction unit andcontacted for liquid—liquid extraction at about 40° C. and 300 psi incountercurrent flow with the extractant. The regenerated extractant (24)would enter the opposite end of the extraction unit. The extractant tobroth flows would be maintained at flow ratios of about 10 to 1. Theextractant would be composed of about 50 wt % tricaprylyl amine, 30 wt %n-octanol, and 20 wt % nonaromatic paraffin with nominal boiling rangeof 180-200° C.

The extraction unit (18) can be any one of many industrially provendifferential or discrete countercurrent extractors which inherently orthrough modification are able to handle the solid slurry of sodiumbicarbonate that is formed during the extraction, and should be of atype constructed to operate under pressures up to 500 psig and bedesigned to give about five equivalent equilibrium states. Three suchextractors are:

1. Discrete stage mixer settlers, each mixer and settler stageconstructed as pressure vessels, and each settler constructed to allowconveying settled solids along the bottom to the discharge pipe, whichis also designed to avoid plugging by solids.

2. Raining bucket contactors, also known as Graesser extractors,constructed as a pressure vessel, with modified internals to allow thesmooth conveyance of a viscous slurry in the aqueous phase.

3. Agitated or stirred column extractors, constructed as a pressurevessel and with modified internals to allow the smooth raining of theslurry. Pulsing of flowor agitation of the column internals is preferredto prevent buildup of solids in the process.

The extractor (18) should be operated such that at least about 90% ofthe incoming lactate values are converted to lactic acid and areextracted into the extractant in one pass. Remaining in the depletedbroth, or raffinate, will be lactic acid, sodium bicarbonate,unfermented sugars, and other broth impurities. Sodium bicarbonate willbe produced in amounts stoichiometrically equivalent to the moles oflactate converted. The final slurry phase raffinate (20) will have up to50 wt % solid sodium bicarbonate after extraction.

Raffinate exiting the extractor (20) will preferably be flashed toatmospheric pressure, with the liberated CO₂ being captured for reuse.The raffinate solids (29) would be removed by centrifugation orfiltration (25), washed with water and would be used to neutralizefermentation. In this way, sodium values are recycled. The aqueousraffinate (26) containing approximately 5-10% of the sodium values, plusall other impurities and lactate values of the raffinate (20), would beused in animal feed formulations. Sodium losses in raffinate should bemade up (13).

Loaded extract from the extractor (18), with lactic acid content ofgreater than about 0.5 mol lactic acid/kg, would be flashed (32) toremove CO₂ (33), and the resultant stream (21) would be back-extractedin about five stages of an extractor train (22) in countercurrentcontact with water (23). This extractant regeneration is operated atabout 140° C. and 100 psig, with water to extractant flow ratios ofabout 2:1. Extractant regeneration can be carried out in one of manyindustrially available extraction devices, including mixer settlers,columns, or centrifugal extractors. The equipment must be designed tohandle the elevated temperatures and pressures.

The regenerated extractant (24) with a lactic acid content of less thanabout 0.1 mol lactic acid/kg, would be saturated with CO₂ at 300 psig(19 c), and added back to the extractor.

The resultant lactic acid stream (28), with a concentration of 15-20 wt%, would be treated with activated carbon, cation exchange, andevaporation, to produce heat stable lactic acid.

Carbon dioxide collected from flashing of raffinate (20) and loadedextractant (33), as well as from fermentation (10) and sodiumbicarbonate decomposition (30) (if used), would be collected, purified,and recompressed for reuse in extraction (19 c).

By the above description, it can be seen that sodium recycles fromfermentation to extraction and back to fermentation. Extractant isrecycled from extraction to extractant regeneration. Carbon dioxide isrecycled from fermentation, bicarbonate decomposition, and flashing toextractant saturation. These three recycle schemes can provide forefficient and cost effective production of lactic acid while reducingthe environmental impact of the production process.

What is claimed is:
 1. A process for the recovery of lactic acid; saidprocess including the steps of: (a) obtaining an aqueous lactatesolution containing lactate salt in solution and a bicarbonate salt; (b)forming a multi-phase system, from the aqueous lactate solution,including an aqueous phase and a water-immiscible, liquid phase; (i)said aqueous phase comprising at least the lactate salt of the aqueouslactate solution; (ii) said aqueous phase having a pH of 4 to 14 andincluding the lactate salt in solution; (iii) said water-immisciblephase including an extractant capable of forming a water-immusciblelactate salt, with a lactate-containing component; (c) extracting saidaqueous phase with said water-immiscible phase by extractinglactate-containing component while forming a water-immiscible lactatesalt with the extractant; (i) said step of extracting being conductedwithout providing the aqueous phase with a pH below 4; (d) separating aresulting water-immiscible phase from a resulting aqueous phase aftersaid step of extracting; and, (e) generating lactic acid from thelactate salt of the extractant; (f) wherein the multi-phase system isformed from an aqueous lactate solution which has been pressurized, byCO₂ addition, to a partial pressure of at least 50 psig.
 2. A processaccording to claim 1 wherein: (a) prior to said step 1(d), removingsolids, in the form of bicarbonate salt, from at least one of: (i) theaqueous lactate solution; and (ii) the aqueous phase of the multi-phasesystem.
 3. A process according to claim 2 wherein: (a) said step ofremoving solids is conducted after a step of pressurizing, by CO₂addition, to at least 50 psig.
 4. A process according to claim 1including the step of: (a) forming and removing bicarbonate solids in atleast one of: the aqueous lactate solution and, the aqueous phase of themulti-phase system.
 5. A process according to claim 4 wherein: (a) saidstep of forming and removing solids comprises precipitation of sodiumbicarbonate.
 6. A process according to claim 5 wherein: (a) said step ofremoving solids comprises removing sodium bicarbonate solids after astep of pressurizing, by CO₂ addition, to at least 50 psig.
 7. A processaccording to claim 6 wherein: (a) said step of removing solids comprisesremoving sodium bicarbonate solids after a step of pressurizing, by CO₂addition, to at least 150 psig.
 8. A process according to claim 1including: (a) said step of removing sodium bicarbonate solids from theaqueous phase, during said step of extracting.
 9. A process according toclaim 8 wherein: (a) said step of extracting comprises conducting amulti-stage extraction process.